Time-division duplex (TDD) methods emulate full duplex communication over a half-duplex communication link. In particular, signals that are communicated from a first device to a second device occur on the same frequencies as, but at a different time than signals communicated from the second device to the first device. Typically, one direction of communication is referred to as the “downlink” direction (and the corresponding signals are referred to here as “downlink signals” or “downlink communications”), and the other direction of communication is referred to as the “uplink” direction (and the corresponding signals are referred to here as “uplink signals” or “uplink communications”). For example, in some systems, separate downlink and uplink timeslots or sub-frames are assigned.
Many systems use TDD for communication. For example, some implementations of the 3rd Generation Partnership Projects (3GPP) Long Term Evolution (LTE) and the Institute of Electrical and Electronics Engineers (IEEE) 802.16 standard use TDD for communication of wireless radio frequency signals.
In order for successful communication between devices in a TDD system, the devices need to synchronize when they switch from communicating in the downlink direction to communicating in the uplink direction and when they switch from communicating in the uplink direction to communicating in the downlink direction. Otherwise, signals will be lost due to interference or missed because each device was not switched to the same signal direction.
In some applications, a distributed antenna system (DAS) is used to relay signals between a first device and a second device in a TDD application. Such DASs, however, typically do not demodulate and decode the RF signals that they distribute. Thus, the DASs cannot determine when to switch between uplink and downlink mode based on the timing information encoded in the RF signals.